With the rapid development of mobile communication technologies, the uses of communication devices such as smartphones, laptops and tablet computers by consumers are increasing; moreover, those electronic devices are becoming more functional while the size of which keeps getting smaller. Along the decrease of volume of electronic devices, the size of electronic components in which are also decreases. However, the requirements for the devices performance and consistency are increased. At present, a capacitive silicon microphone is a microphone fabricated by a surface (e.g. silicon substrate) processing technology or a bulk silicon processing technology. The surface processing technology or the bulk silicon processing technology is compatible with integrated circuit fabrication process. Moreover, the size of the microphones may become very small by using a miniaturizing CMOS process technology, thus being widely applied into portable electronic products such as mobile phones, laptops, Bluetooth headsets, and cameras.
Refer to FIG. 1, a MEMS microphone includes a silicon substrate 10, an back cavity 101 which is up-down through-cut of the substrate 10, a parallel plate capacitor which is set on the substrate 10 and is constituted of an upper polar plate 103 and a lower polar plate 102. The lower polar plate 102 is usually a fixed polar plate, the upper polar plate 103 is a vibrating diaphragm of the microphone, and there is an air gap 104 formed between the lower polar plate 102 and the upper polar plate 103 as an insulating dielectric of the parallel plate capacitor. A supporting body 105 is set on the periphery of the upper polar plate 103 supporting the upper polar plate 103, and a plurality of release holes 106 are set on the upper surface of the upper polar plate 103 to volatilize dielectric material filled in the air gap 104 during the fabrication process. The upper polar plate 103 of the parallel plate capacitor vibrates due to external outside acoustic signal, which causes changes in the distance between the upper polar plate 103 and the lower polar plate 102, thus to change the capacitance of the parallel plate capacitor, generates a voltage signal and so as to realize the acoustic-electric conversion function.
In practical production, polycrystalline silicon films are usually adopted as the upper polar plate and the lower polar plate of the MEMS microphones. The polycrystalline silicon films are generally grown by Low Pressure Chemical Vapor Deposition (LPCVD) process. There is an internal stress gradient difference problem between different areas of the polycrystalline silicon film. Moreover, there is a distinct difference in internal stress of vibrating diaphragm of silicon microphone chips in different production batches, which can influence the consistency of process performance and product quality. On the other hand, if the stress of the polycrystalline silicon film is released insufficiently, it will cause an over large background noise, and if the range of mechanical vibration of the vibrating diaphragm is small, it will cause low sensitivity of the MEMS microphone.
Therefore, in the IC industry, it is desired to obtain a novel capacitive silicon microphone structure and a fabrication method thereof, which would release the stress effectively, enhance the structure sensitivity and overcome non-uniformity problem of the stresses.